For many precision engineering systems, it is required to measure the position of an object to nm-level tolerances over long periods of time, e.g., days, months, or even years. The range of motion of the object may be small, e.g., less than a mm in any direction, but should be controlled with an active servo loop for stability and to provide for fine, controlled adjustments.
An example of such a precision engineering system is the exposure system of a modern photolithography tool. Examples of exposure systems are described in Microlithography: Science and Technology, edited by J. R. Sheats and B. W. Smith, Marcel Decker, Inc. (New York, N.Y. 1998). Exposure systems include illumination assemblies to illuminate a reticle or mask and projection objective assemblies to image the reticle pattern onto a wafer.
In such exposure systems, various components such as a wafer stage, a reticle stage, and optical components such as lenses and/or mirrors and/or subassemblies thereof, are held in relationship to each other and/or in relation to a frame. Moreover, certain components are configured to move relative one another or the frame and such movement needs to be monitored by high-accuracy sensors, such as displacement measuring interferometers.